1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic type image forming apparatus and an image forming method thereof.
2. Description of the Related Art
Image quality of output images formed by recent image forming apparatuses has significantly improved. Thus, demands for higher image quality control by the user are becoming greater. Nevertheless, image forming apparatuses of an electrophotographic type using an electrostatic process face a problem of changes of image quality due to, for example, environmental changes (e.g., temperature, humidity) and degradations with age (e.g., degradation of toner). Particularly, change of toner density is a problem in a case of forming monochrome images. Furthermore, in addition to change of toner density, change of color reproduction, change of gradation, and change in the amount of color registration are problems in a case of forming color images.
As a commonly used method for resolving such changes of image quality, there is, for example, a method of forming an output image based on image data dedicated for printing along with forming an image based on a relatively small pattern(s) dedicated for image quality management (hereinafter also referred to as “reference image”) on a photoconductor and/or a image transfer medium, measuring a physical quantity (e.g. amount of adhered toner, gradation, amount of color registration) regarding the image quality of the reference image by using a sensor, and controlling image forming conditions (e.g., electric potential for charging a photoconductor, amount of light to be emitted to the photoconductor, developing bias, amount of development toner to be supplied) based on values obtained by the measurement of a physical quantity. With this method of controlling image quality, changes of image quality can be precisely controlled with high accuracy. In a case where the image quality controlling method using the reference image is performed by an image forming apparatus that forms an image on a plain paper (cut-sheet) sheet by sheet such as on A4 size paper, the reference image is formed in an area between output images on a photoconductor drum or a transfer belt, to thereby measure the physical quantity and control various image forming conditions (see, for example, Japanese Laid-Open Patent Application No. 7-181795). On the other hand, in a case where the image quality controlling method using the reference image is performed by an image forming apparatus that forms an image on continuous form paper, the reference image is formed in an area outside of an output image forming area (non-output image forming area) since the output image forming area is constantly used for printing an output image (see, for example, U.S. Pat. No. 5,124,732).
In a case where an output image is continuously formed, for example, a case of forming an image on continuous form paper on an intermediate transfer belt, the surface conditions of the intermediate transfer belt vary between its output image forming area and its non-output image forming area. The output image forming area of the intermediate transfer belt is constantly in contact with a recording medium (sheet) and subject to friction and changes of charge, whereas the non-output image forming area does not contact a recording medium (sheet) and is subject to relatively moderate conditions. Therefore, in a case of forming the same image in the output image forming area and the non-output image forming area, the image formed in the output image forming area and the image formed in the non-output image forming area may not have the same image quality depending on the operating state of the image forming apparatus. Thus, in a case where there is a significant difference of measured image quality between the output image formed in the output image forming area and the reference image formed in the non-output image forming area, the image quality of the output image formed in the output image forming area cannot be sufficiently controlled even if control efforts are based on data of the physical quantity obtained from the reference image formed in the non-output image forming area.
When forming (printing) an image on a continuous paper where its image quality is controlled by forming a reference image in an output image forming area for controlling image quality with high precision, it becomes necessary to interrupt the continuous printing process. This interruption of the printing process lowers printing efficiency particularly in a case of printing large amounts of continuous paper at high speed.
Therefore, in a case of forming large amounts of images on a continuous paper at high speed, it is difficult to achieve both precise monitoring of image quality of an output image being printed and forming a reference image used for the image quality monitoring while forming the output image.